- "When water was plentiful on my world we could outswim all others."
- —Onox recalling the past Realm of Twilight.
Onox is a tall reptilian creature known as a Dark Dragon, with green scales and two large winglike fins on his back. He has luminescent green slit-eyes and a snakelike mouth. For unknown reasons, Onox's physiology is radically different from the other known Dark Dragon.
Onox is an extremely blunt, pragmatic, and patient being, likely due to his long lifespan and isolation as among the last of his kind. In battle he is brutal and utterly merciless. He is not entirely without empathy, however, as evidenced by his taking-in of a dying Veran and nursing her back to health. Onox is notably nontalkative, only speaking when directly addressed.
Thanks to his powerful strength and fins Onox is an excellent swimmer, capable of easily matching a Zora in underwater agility and speed despite his massive size. He is also capable of producing pink energy beams from his mouth in either a fireball-like projectile or cone of searing light.
This section contains plot details.
Onox was born prior to the creation of Hyrule, in an age when water was plentiful in the Realm of Twilight. He still remembers in this age, his people, the Dark Dragons, could outswim everybody else in the planet. As a child, Onox witnessed his homeworld falling victim to the Druthulidi Bellum, who killed most of its biosphere, causing the world to rapidly freeze and desertify, eventually turning into a dark, bleak wasteland.
Following the arrival of numerous prisoners of war through the Mirror of Twilight, one Fairy named Veran, who was badly injured, wandered off from the others into the wilderness to die. Onox finds this Great Fairy and senses that she had great and powerful magic, so he shelters her from the harsh Realm of Twilight and helps her recover from her wounds. Because of this the two develop a close and possibly romantic relationship. After this Veran, Morsheen and Una establish the Twili Empire.
After the three Great Fairies murder the Twili King Mizorant and place Midna on the throne, Onox helps both Midna and Veran round up and imprison the Bonemold Monks and their leader Lady Middee in Valran for causing riots.
When Prince Zant starts a rebellion to overthrow Midna and captures Una, Veran invites Zant's forces to Uzu where the Fairies are based, then returns to Uzu, where Onox and Morsheen are waiting for her. When Zant's army assaults Uzu, Onox commands his army alongside a contingent of Majora's spawn against Zant. After Zant defeats this army and reveals that he is planning to invade Hyrule Veran swears allegiance to him, so that she could return there.
Onox assists Zant in his invasion of the Palace of Twilight, where they successfully manage to overthrow Midna. Onox then begins to prepare the Twili armies for the invasion of Hyrule.
Shortly before the Twili's march into Hyrule, Zant's Parella associate seeks out Onox and tells him of her plan to poison the Zora River, an important water source for the Hyruleans, so they can reduce resistance to their future invasions. Onox, unaware that this creature is a servant of the Druthulidi which devastated his home so long ago, accepts, and the Parella gives him the poison.
The Gerudo Wars
With the portals to Hyrule complete, Zant, Veran, Onox and the Parella open an entrance near Domain Prime, capital of the Zora Dominion, near the river bank. The Parella explains her plan to Zant, Veran and her servant Princess Sokuuf, and instructs Onox to swim upstream to the source of the river and dump the poison while Zant's Twili and Sokuuf's River Zora keep the Dominion busy.
However, inside the caverns under the Zora's Domain, Onox is intercepted by Princess Ruto, Zaleen and Laruto. He attempts to outswim them and head to the river's source, but Laruto manages to grab his armour and drives him straight into an ice shard fired by Ruto, which promptly impales and kills him. Zant orders a retreat, but Veran refuses to leave without Onox and becomes stranded on Hyrule after the Mirrors of Twilight are destroyed.